Developing an integrated approach for the performance verification for Large Coordinate Measuring Machines (CMM) at the Standards and Calibration Laboratory (SCL)

Abstract

To support local industries, the Standards and Calibration Laboratory (SCL) in Hong Kong has developed a performance verification service for Coordinate Measuring Machines (CMM) meeting the requirements of ISO 10360-2:2009 ("the ISO standard"). In the ISO standard, a non-contact laser-based approach could be used in generating long test length. In that approach, additional steps of measurements for an artifact with normal coefficient of thermal expansion (CTE) ("Requirement A") and bi-directional measurements ("Requirement B") are required. When the time and effort spent on the laser alignment are taken into consideration, using the laser-based approach with a laser interferometer for on-site CMM verification might not be justified. To address these issues, SCL has developed an integrated approach. In this approach, most of the test lengths would be measured with a step gauge. For test length exceeding the coverage of the step gauge, the step gauge would first be fitted and have the step lengths measured. Test lengths longer than the step gauge would then be measured by using laser interferometer. The benefit of such approach is three-fold. Firstly, Requirement A can be fulfilled by measuring a longer distance of the step gauge. For Requirement B, any of the 20 mm step length on the step gauge can be used in lieu of the short gauge block. Secondly, the step gauge could guide the fitting of the laser optics thereby greatly minimize the alignment time and efforts, in particular for retro-reflector on some measuring heads that do not support automatic angle setting. Lastly, with the measurements for Requirements A and Requirement B, and the unidirectional measurement using laser interferometer all lie on the same axis, they can be cross-checked to minimize any setting and cosine errors. The approach could promote the use of the ISO standard by not just complying with its requirements but also providing a more effective, comparable and rigorous way in carrying out the performance verification for CMM.